I don't understand why i haven't succed whith this small button. But you are right. It is this button.
Thanks for your help.
For the 'crap' on the watch, it is because i have brushed the case.
And it seems that some 'crap' rest was on the cloth where i put the watch.
And maybe static electricity have made this.
After taked picture, i have blowed on it, and it seems to be ok.